1. Field of the Invention.
The present invention relates to crystal growth, and, more particularly, to epitaxially growth of semiconductors.
2. Description of the Related Art.
Epitaxial growth of electronic grade semiconductors such as gallium arsenide (GaAs) relies on methods such as liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD). MBE and MOCVD both provide the ability to grow extremely abrupt p-n junctions and heterojunctions of lattice-matched materials; but the best performance low noise and power monolithic high frequency (60 GHz) radar modules are fabricated from GaAs and Al.sub.x Ga.sub.1-x As by MBE. However, MBE has serious shortcomings, including the relatively small effusion cell capacity. Typically, less than one hundred epilayers can be prepared between recharges of the cells, and frequent recalibration of the sources are required to compensate for depletion of the cell charges. Recently chemical beam epitaxy (CBE) has been proposed as a system to overcome these shortcomings by combining features of MBE and MOCVD; see, W. Tsang, Chemical Beam Epitaxy of InP and GaAs, 45 Appl. Phys. Lett. 1234 (1984).
The CBE system of Tsang (FIG. 1 is a schematic illustration) has the basic hardware structure of an MBE system: a hemispheral vacuum chamber with sources arranged on the curved surface and aimed at the substrate holder located at the chamber center. The sources in MBE systems are effusion cells containing solid or molten elements (for example, growth of layers of Al.sub.x Ga.sub.1-x As with various x values and with silicon for n doping requires a cell for each of aluminum, gallium, arsenic, and silicon). The CBE system replaces some or all of the conventional elemental effusion cells with cells that are outlets of tanks of metalorganic gasses (for example, trimethylaluminum (TMAl), triethylgallium (TEGa), and trimethylarsine (TMAs)). This substitution solves the MBE problems of effusion cell life and flux drift, and the CBE cells are much simpler than the MBE effusion cells; but the problem of incorporation of carbon from the gasses in the epilayers arises. See E. Tokumitsu et al, Molecular Beam Epitaxial Growth of GaAs Using Trimethylgallium as a Ga Source, 55 J. Appl. Phys 3163 (1984). This carbon incorporation results in GaAs buffer layers whose carrier density is unacceptably high (in the order of 1.times.10.sup.16 carriers/cm.sup.3) for use with high frequency radar modules; carrier density of at most 1.times.10.sup.14 /cm.sup.14 is required.
Conventional MOCVD has less of a problem of carbon incorporation because of the presence of atomic hydrogen at the growing epilayer surface; the atomic hydrogen is generated by the decomposition of arsine. This atomic hydrogen reacts with methyl radicals to form relatively inert methane molecules. Under the line of sight conditions of CBE, however, thermal decomposition of arsine at the epilayer surface is unfavored. Precracking of arsine has been used to overcome this problem, but the species impinging on the epilayer are arsenic and molecular hydrogen, not atomic hydrogen. Thus carbon incorporation is a problem even with the arsine precracking approach; see W. Tsang, Chemical Beam Epitaxy of InGaAs, 58 J. Appl. Phys. 1415 (1985).
One approach is to provide a hydrogen ambient (about 3.times.10.sup.-4 3.times. 10.sup.-4 Torr) for MBE or CBE growth and add a hot filament adjacent the growth surface to dissociate the ambient hydrogen into atomic hydrogen; see R. Bachrach et al, On the Possibility of MBE Growth Interface Modification by Hydrogen, 1 J. Vac. Sci. Tech. B 142 (1983). But this approach is not suited to growth on large diameter substrates.
Carbon incorporation in the growing epilayers is a continuing problem in the known CBE systems.